1. Field of the Invention
This invention relates to flow sampling systems and, more particularly, to proportional gas flow sampling method and apparatus utilizing a channeled flow splitting device.
2. History of the Prior Art
The prior art is replete with systems adapted for taking proportionally constant samples from flowing gas streams in gas lines. The purposes of such samples is to make measurements of various types representative of the main gas flow. Proportionality of the sample is important in order to avoid variable calibration factors, and the challenge is to provide equipment which takes a proportionally constant sample over an appreciable range of flow rates in the main gas line. The methods and apparatus set forth and described in U.S. patent application Ser. No. 06/272,204 filed June 10, 1981, now U.S. Pat. No. 4,396,299, and assigned to the assignee of the present application sets forth one embodiment of such a device. While the present invention is useful in other applications as well, the equipment disclosed in said patent application forms a good illustration of flow measurement systems.
Other prior art systems have specifically addressed the division of flow for constant proportionality measurement. For example, co-pending U.S. patent application Ser. No. 06/325,312 filed Nov. 22, 1981, now U.S. Pat. No. 4,446,748, teaches such a system wherein a baffle arrangement is provided in a main gas flow line such that a constant proportion of the total flow enters a by-pass line upon diversion by the baffle. It is known in the prior art to use just baffles for proportional flow sampling, at least over a narrow range of flow rates. A sampler relying solely on baffles has a very simple piping arrangement, and an advantage of the baffle system is that little pressure drop is introduced into the main line. A disadvantage of the baffles is that the proportional cut should not be less than 10-20% of the total flow. If one tries to extract too small a fraction, then changes in the flow profile as a function of Reynolds number will have a significant effect and reduce accuracy. Other prior art systems incorporate a sampler relying solely upon an orifice arrangement without baffles. Such an arrangement is disclosed in the abovementioned patent application Ser. No. 06/272,204. An advantage of the orifice sampling system is that a very small cut can be taken from the main line, such as one part in one thousand.
A disadvantage of either a baffle system or an orifice plate system by themselves is that the dynamic range is limited in an energy flow meter application. In the latter instrument, a proportional flow sampler is combined with a modified titrator-type apparatus for determining energy content or calorific value of the kind generally disclosed in U.S. Pat. Nos. 3,777,562 issued Dec. 11, 1973; 4,062,236 issued Dec. 13, 1977; 4,125,018 issued Nov. 14, 1978; and 4,125,123 also issued Nov. 14, 1978. In the energy meter section of such equipment, the sample flow is mixed with air and burned. There will only be about a five-to-one range of flows that can be accommodated with a single burner design and the greater the flow range at the burner, the less the inherent accuracy will be.
Other prior art approaches to proportional flow sampling have addressed the critical aspect of obtaining representative samples through flow division and pressure regulation. For example, U.S. Pat. No. 3,377,867 to Nitescu sets forth an automatic sampling device comprising a flow divider and pressure stabilizing means for equalizing the flow within the channels of the flow divider. This conventionally accepted method of flow division affords automatic sampling for a multiphase, non-homogenous fluid. The flow division principle incorporates an orifice plate and channel block having a plurality of channels of identical dimension for permitting flow therethrough. The flow through one channel is, however, fed to a by-pass line through a direct tap coupling, and the flow therethrough is regulated by a zero differential flow controller. The flow controller nullifies any pressure differential between the valve and the exit whereby flow through the by-pass line will be identical to the flows through the other channels. Such differential flow regulation is an integral part to proper flow sampling and is particularly critical to sampling units of the type described above utilizing an isolated section of diverted flow. Disadvantages of such methods and apparatus include the inherent limitations to the possible flow splits for sampling. The division of flow is obviously directly related to the number of identical channels through which flow is permitted to pass. The availability of high split ratios is therefore limited in such systems to the number of channels which can be provided. This limitation can become critical in instances of ordinary flow variation.
Another prior art approach is shown in U.S. Pat. No. 3,930,414 to Russel wherein a variable flow restriction in the sample line is provided to maintain proportionality to the pressure drop across a flow restricter in the main line. Certain advantages exist for such proportional flow sampling systems. The variable flow restriction system has been shown to be effective in establishing flow regulation without the use of sectioned flow dividing units. Such systems utilize main line orifice plate assemblies and differential pressure sensors thereacross for establishing flow proportionality. Certain disadvantages remain, however, relative to flow sampling ratios and accurate proportionality during flow rate variations without the utilization of a flow divider.
It would be an advantage, therefore, to incorporate certain ones of the advantages of a flow dividing unit with an upstream sampling unit whereby a proportional flow sample may be obtained with maximum efficiency and accuracy and a minimum of complex hardware coupled thereto. The methods and apparatus of the present invention provide such a system by incorporating channeled plugs in the main flow line and an upstream sample line for obtaining proportional flow sampling. The system is also constructed wherein the average cross-sectional area per channel in the main line plug is substantially greater than the average cross-sectional area per channel in the sample line plug, while the hydraulic diameters of each are relatively equivalent, or within pre-defined ranges. In this manner, a fixed fraction of the flow into the sample line is maintained regardless of overall flow rates and wherein an advantageously high split is possible without the necessity of a plug having an equal number of apertures corresponding to the flow split ratio.